Combination of cdk4/6 and mtor inhibitors suppressed doxorubicin-resistant osteosarcoma in a patient-derived orthotopic xenograft mouse model: A translatable strategy for recalcitrant disease

Background: Osteosarcoma is the most frequent malignant bone neoplasm. The efficacy of combination therapy of a cyclin-dependent kinase 4/6 (CDK4/6) inhibitor and a mammalian-Target-of-rapamycin (mTOR) inhibitor was previously reported in several cancer types. In the present study, we evaluated the efficacy of a combination of palbociclib (CDK 4/6 inhibitor) and everolimus (mTOR inhibitor) on an osteosarcoma patient-derived orthotopic xenograft (PDOX) mouse model. Materials and Methods: osteosarcoma PDOX mouse models were randomized into five treatment groups of seven mice each: Group 1, untreated control; group 2, doxorubicin treatment; group 3, palbociclib treatment; group 4, everolimus treatment; group 5, palbociclib everolimus combination treatment. Treatment duration was 2 weeks. Results: The palbociclib everolimus combination reduced the tumorvolume ratio in the osteosarcoma PDOX mouse model compared with the control and doxorubicin (p=0.018). Everolimus alone also inhibited osteosarcoma PDOX growth compared to the control (p=0.04), but less than the combination. Palbociclib alone and doxorubicin were ineffective. There were no significant body-weight losses in any group. Only the palbociclib everolimus combination induced extensive tumor necrosis observed histopathologically. Conclusion: The present study demonstrated that the combination of CDK4/6 and mTOR inhibitors can be a translatable approach for doxorubicinresistant osteosarcoma in the clinic.